1. Field of the Invention
This invention relates to a radiation-image reading apparatus which reads a recorded radiation image from a radiation-image recording medium formed by using, as a means for recording radiation images, a phosphor layer which absorbs or accumulates radiation. More particularly, this invention relates to an improvement of a type of radiation-image reading apparatus which reads a recorded radiation image by scanning the phosphor layer of the radiation-image recording medium with excitation light and detecting emission from the phosphor layer on the basis of a method of utilizing a phenomenon in which, if a phosphor layer is irradiated with excitation light, accelerated phosphorescence emission takes place at the phosphor layer in accordance with the intensity of the accumulated radiation.
2. Description of the Related Art
This type of radiation-image reading apparatus has been developed in place of a method based on a radiograph using silver salt with a view to avoiding problems, e.g., exhaustion of silver resources. There are examples of this type of radiation-image recording apparatus, such as the ones disclosed in Japanese Patent Laid-Open Nos. 59-13235 and 59-13236.
The apparatus described in these publications is provided with: a radiation-image recording member which records a radiation image by using a phosphor layer which absorbs and accumulates radiation and which emits light in accordance with the intensity of the accumulated radiation after it is irradiated with excitation light; a light source which generates the excitation light; and an emission detecting device which detects the emission from the phosphor layer by using a light reflecting member disposed in the path of the excitation light and receiving light generated by the emission from the phosphor layer (hereinafter referred to as "emission light") at a position deviated from the path of the excitation light. The phosphor layer is scanned with the excitation light, and the states of emission at the time of scanning at respective points on the scanning line are successively detected by the emission detecting device, thereby reading the radiation image recorded on the radiation-image recording member.
To improve the efficiency of reading, it is desired, in the system of scanning using the excitation light, to suitably design the shape of the radiation recording member and the method of moving the irradiation spot of the excitation light.
The above-mentioned publications disclose a technique of using a cylindrical phosphor layer, making this layer rotate about the axis thereof, and relatively moving the phosphor layer and the source of excitation light in the direction of the axis of the phosphor layer, and another technique of using a flat-plane phosphor layer and moving the irradiation spot longitudinally and latitudinally along the plane.
With respect to the shape of the radiation recording member and the method of moving the irradiation spot of the excitation light, however, it is also necessary to consider the adaptability to the outline of the radiation image recorded on the radiation recording member. If the shape of the recording member and the moving method are not designed with consideration for the outline of the radiation image, a useless part of the scanning increases so that the time taken to detect the outline of the image read and recognized is lengthened, resulting in a reduction in the efficiency of the reading operation, or the extent of movement of a movable part for use in the scanning increases so that the space occupied by the space is increased.
In general, conventionally, a dichroic mirror is used as the light reflecting member.
The light emitted from the phosphor layer is very weak compared with the excitation light. Therefore, to make the detection of emission more accurate and positive and increase the accuracy in the reading of the image, it is essential to minimize the loss of emission light in the path to the emission detecting device and thereby increase the amount of emission light reaching the emission detecting device while preventing strong light such as refection light from entering the emission detecting device.
A dichroic mirror was selected as the light reflecting member in the above-mentioned examples and was used as a measure to satisfy this requirement.
A dichroic mirror can provide a wavelength selecting property of allowing transmission of excitation light while reflecting emission light different from the excitation light. Therefore, it can eliminate reflection light from the excitation light by simply allowing transmission of this reflection light while it reflects light emitted from the phosphor layer to the emission detecting device.
However, the dichroic mirror tends to cause scattering and absorption or refraction due to its specific structure. In an apparatus which makes use of the dichroic mirror, there is a possibility of the emission light being reduced by scattering and absorption at the time of reflection, resulting in difficulty in detection of very weak emission, or there is a possibility deviation of the focal point during spot irradiation of the excitation light due to refraction at the time of transmission through the dichroic mirror, resulting in difficulty in performing reading with desired degrees of accuracy and definition. In particular, it is difficult to use the dichroic mirror in a case where a diffraction image obtained as a radiation image is very weak and where determination of the crystal orientation becomes incorrect unless the position of a diffraction spot can be read with accuracy, such as a case in which a crystalline substance is irradiated with X rays and a diffraction image thereby obtained is observed to examine the crystalline structure of the substance.